It is common in the processing of silicon wafers to remove or etch portions of a layer of silicon dioxide using an etchant. The common etchant is hydrofluoric acid. This is typically done using an immersion process wherein a wafer, or one or more carriers containing a number of wafers, is immersed in the desired hydrofluoric acid containing processing fluid.
One of the disadvantages of immersion etching processes is that the wafers typically exhibit an increase in the numbers of particulates which become adhered to or imbedded in the wafer. As the feature size of semiconductor wafers continues to decrease into the sub-micron sizes, now typically in the range of 0.1-0.5 micron, the need for minimizing contamination becomes more acute. These small feature sizes also create additional problems for immersion processing because surface tension effects can reduce etching uniformity and resulting product quality.
It has previously been recognized that silicon wafers can be processed by using etchant gases, including gases which contain hydrofluoric acid (HF). In an article entitled, "Etching of Thin SiO.sub.2 Layers Using Wet HF Gas" authors P. A. M. van der Heide et al., describe the etching of silicon dioxide layers by using vapor mixtures of HF, water and nitrogen. The article describes using a flow of nitrogen carrier gas through a vessel containing a 10% hydrofluoric aqueous solution. The nitrogen carrier produced a flow of etchant gas which was directed through a nozzle against the surface of a small silicon wafer being etched. A flow of dry nitrogen was passed over the opposite side of the wafer to reduce the effects of ambient atmospheric water which was present in the essentially open atmospheric process. Temperatures from 25.degree.-40.degree. C. were indicated for the HF solution and from 25.degree. C. to about 60.degree. C. for the wafer. The authors also reported that high temperature treatment under vacuum conditions following the etch provided complete removal of oxygen.
One deficiency of the process described is the continuation of etching after the process exposure has begun. This results in uneven etching rates and problems in the resulting integrated circuit devices due to variations in feature sizes across the device. The etching uniformity is also deficient in that the process arrangement may not provide assurance of equilibrium homogeneous presentation of the reactant gas to the wafer being processed. Variations in the uptake of the vapors by the incoming carrier gas stream can result in instantaneous variations in the etchant gas stream which can affect processing. This deficiency may not have been as significant in the reported processing because of the substantial diluting effect of the carrier gas which also resulted in very slow etching rates.
Another serious deficiency of the processing described by van der Heidi is that the etching rates are very slow and accordingly not practical from a commercial processing standpoint where high volume production rates must be maintained. The authors describe the need to produce an initial wetting time and then periods of 3.5-5 minutes were reported as acceptable for removing very thin layers of only 1.2-3.5 nanometers. The resulting indicated etching rates were about 1 minute per nanometer (1 minute per 10 Angstroms). These relatively slow etching rates would provide commercially unfeasible processing times of 5-100 minutes where layers of 50-1000 Angstroms are to be removed. Such processing times are sufficiently slow to prevent acceptance of the van der Heide vapor processing as a substitute to the faster processing times possible using the prevalent immersion processing.
U.S. Pat. No. 4,749,440 to Blackwood et al. shows a gaseous process for removing films from substrates which utilizes an anhydrous HF supply. The process is described to involve passing a flow of dry nitrogen over the wafer and then introducing a flow of reactive gas which is preferably an anhydrous hydrogen halide gas such as anhydrous hydrogen fluoride gas. A flow of water vapor laden nitrogen is also passed over the wafer before the anhydrous HF gas flow is begun and this is continued until after the anhydrous HF gas flow has been stopped. This type of processing leads to nonhomogeneous mixing occurring during the brief processing period of approximately 5-30 seconds. During the initial period exclusively water laden carrier gas is first introduced. This subjects the surface of the wafer to a high gradient moisture increase over a very short period of time. This is immediately followed by anhydrous HF gas which is incapable of reaching any effectively uniform or equilibrium condition with the water vapor because of the prior introduction of the water vapor and the quickly terminated introduction of the HF which creates a highly reactive combination varying from point to point across the wafer surface. The resulting highly reactive but nonhomogeneous etchant gas is capable of high etch rates. Unfortunately, the results using this process have proven to be highly variable with nonuniform etching on the same device being a common problem as well as nonuniformity from one device to the next. Acceptance of this processing system was initially enthusiastic but has been nearly abandoned by chip manufacturers because of the seriousness of the nonuniformity problem in etching rates.
The nonuniformity problem also results from variations in the amount of water present in the matrix of the material being processed which can have very significant effects on the effective localized HF-water concentrations on the surface of the chip during the high speed etching reaction which occurs during this type of processing. Wafers otherwise processed in a similar manner may exhibit highly differing etch rates merely because they have been allowed to sit for hours under ambient conditions thereby uptaking atmospheric moisture to a substantially greater degree than other wafers processed soon out of a furnace or other moisture eliminating processing. Such variations in moisture content of the wafers are typical and any special preatmospheric treatment necessarily increases processing time, processing logistics, or both.
Another more recent approach to vapor etching of wafers using HF is incorporated into the processing machine referred to as the EDGE 2000 from Advantage Production Technology. This system utilizes a specially configured processing chamber which vacuum treats the wafer prior to processing with the etchant. This approach attempts to remove residual moisture from the wafer to address the nonuniform etching rate problem discussed above. The relatively short duration vacuum processing cannot remove all moisture content variations. The wafer is exposed to a highly reactive HF-water gas stream which is directed at the wafer in a vertical orientation from one or both sides. The jet of incoming reactant gas impinges upon the surface or surfaces of the wafer at localized central areas and typically results in non-uniformities in mass transfer due to this localized impingement despite the vacuum processing directed toward water removal.
Thus there remains a strong need in the art for gaseous HF processing which will provide uniform and repeatable results with high effective etching rates while achieving low contamination and particle counts. Other objectives and advantages of the invention are also indicated herein.